Analyzers such as spectrophotometers use deuterium lamps as light sources for detecting transmittance and absorbance of ultraviolet/visible light in the 180 to 400 nm wavelength band.
Deuterium lamps place a window made of UV glass or quartz glass that allows ultraviolet light to pass through a part of a glass bulb and are able to emit light in the ultraviolet/visible range from that window by applying the voltage of the drive device to electrodes formed within the bulb (i.e., across the cathode and anode) to produce and maintain the electrical discharge.
Deuterium lamps used in analyzers require stability in the amount of light emitted through the window in order to provide stable measurement data. Because the stability of the amount of emitted light depends on the stability of the drive current of the deuterium lamp, lamp drive devices use power supply circuits with a constant current source (Patent Literature 1).
FIG. 3 is a schematic block diagram of a spectrophotometer that uses a deuterium lamp drive device described in Patent Literature 1 as the lamp drive device.
Main power supply 3 supplies drive circuit 7 with a commercial 100 V (or 200 V) AC voltage. Drive circuit 7 comprises: a rectifier circuit 4, which transforms AC voltages to DC voltages; a deuterium lamp 1 (discharge tube); a constant current source 2, which supplies constant current to deuterium lamp 1 (during discharge); a heater power supply 6, which heats the cathode of deuterium lamp 1; and trigger voltage generator 5 (trigger power supply), which temporarily applies a pulsed trigger voltage (about 350 V) to start discharge.
With this lamp drive device, the cathode of deuterium lamp 1 is heated by heater power supply 6 so that it discharges thermions.
Then a DC voltage higher than the self-sustaining discharge voltage (about 80 V) required to maintain discharge is applied to the electrodes before the start of discharge by power supply circuit 7a, which comprises rectifier circuit 4 and constant current source 2. After discharge starts, a stable discharge is maintained by supplying a constant current from constant current source 2.
To start a discharge, a lighting instruction signal from a controller 9 (the controller of the computer that controls the entire spectrophotometer also serves as the lamp drive device controller) of the lamp drive device is used to apply pulsed trigger voltage (e.g., 350 V) from trigger voltage generator 5. This starts a discharge.
Then, a determination is made as to whether the lamp is lit by using a voltage monitor circuit 8 to measure the voltage applied to the electrodes of deuterium lamp 1 from power supply circuit 7a (rectifier circuit 4 and constant current source 2). This determination is made so that no measurement data is acquired in error with the lamp unlit should the lamp fail to light despite the application of the trigger voltage.
An example of the voltage monitor circuit 8 includes a voltage divider resistor for detecting electrode voltage and an AD converter to convert the voltage value measured by the voltage divider resistor into digital data, which is sent to controller 9.
The manner in which the voltage monitor circuit 8 determines whether or not the lamp is lit is described next. The voltages applied to the electrodes by trigger voltage generator 5 and heater power supply 6 are irrelevant to the lighting status determination and is not described here.
First, prior to the start of discharge, no current is flowing to deuterium lamp 1. The voltage monitor value A measured by voltage monitor circuit 8 when not discharging is the value determined by voltage α applied to the electrodes by power supply circuit 7a (rectifier circuit 4 and constant current source 2) and the error α in voltage monitor circuit 8, which measures the electrode voltage. “Error α in voltage monitor circuit 8” is produced by the fact that individual voltage monitor circuits 8 used in individual lamp drive devices can be different and not exactly identical. Voltage monitor value A is treated as including an error α within a set range defined by the specifications for each voltage monitor circuit 8 of each respective device.
Once discharge begins (after lighting), a constant current flows to the electrodes, and the self-sustaining discharge voltage for maintaining discharge is produced in the electrodes. The self-sustaining discharge voltage b required for flowing a constant current to the electrodes is a value unique to each deuterium lamp 1 (initially, about 80 V) and may increase with aging.
The voltage monitor value B measured by voltage monitor circuit 8 after discharge begins is the value determined by the self-sustaining discharge voltage b of deuterium lamp 1 and the error α in voltage monitor circuit 8.
Accordingly, the determination of the lighting status is made by determining whether the measurement by voltage monitor circuit 8 shows a voltage monitor value A (A is equal to the voltage value a that is applied when not discharging further increased or decreased by error α in the voltage monitor circuit) or voltage monitor value B (B is equal to the self-sustaining discharge voltage value b increased or decreased by error α in the voltage monitor circuit).
Traditionally, this determination has been made by selecting a threshold value S such that:Voltage monitor value A>Threshold value S>Voltage monitor value B  (1)The threshold value S was a value that was selected independently of the device used. A determination of lit or unlit was made by comparing the result of the measurement against the threshold value S and seeing whether it was greater than or less than the threshold value S.